The present invention concerns a catalyst for steam reforming of methanol, the production of the catalyst according to the invention and the use thereof.
In the steam reforming of methanol, on the catalyst according to the invention, methanol (MeOH) and steam (H2O) are converted on the catalyst to give carbon dioxide (CO2) and hydrogen (H2) in accordance with the following reaction:CH3OH+H2O→CO2+3H2  (i)
The division of methanol into carbon monoxide (CO) and hydrogen occurs as a secondary reaction:CH3OH→CO+2H2  (ii)
The production of hydrogen as an energy carrier is ever increasing in importance because of the decreasing availability of fossil fuels and for ecological and economic reasons. Obtaining energy from hydrogen by combustion in fuel cells is one of the most important applications.
The production of hydrogen from methanol by steam reforming of methanol is effected in a methanol reformer which can either be used in a static mode or in a mobile mode. In the methanol reformer a methanol-water mixture with a water-methanol ratio of between about 1.3 and 1.5 is compressed to up to 20 bars, evaporated, superheated to between about 250 and 280° C. and converted in accordance with the above-indicated reactions. The hydrogen is separated off by means of a hydrogen-permeable membrane catalytically or by pressure swing adsorption.
Mobile methanol reformers are used for example as mobile hydrogen sources for fuel cells in order not to have to carry the hydrogen in pressure gas tanks for safety reasons. Methanol has the advantage, as the starting material for mobile hydrogen production, that it can be carried along in the form of a liquid fuel and is thus safer than hydrogen although methanol is also not completely harmless to handle by virtue of its toxic nature and combustibility. Because of its high hydrogen-carbon ratio and its low sulfur content methanol is also highly suitable as a hydrogen source and can be produced from renewable sources such as for example biomass in environmentally fashion and independently of fossil fuel sources.
Two basic demands on a catalyst are a high catalytic activity and high selectivity in respect of the desired product or products. In the steam reforming of methanol a high catalytic activity is demonstrated by high conversion rates, in particular at temperatures which are as low as possible. Good selectivity of a catalyst for steam reforming of methanol is distinguished in that the product of the reaction of methanol and water in accordance with the desired primary reaction (i) has a high proportion of carbon dioxide and hydrogen and a proportion of carbon monoxide, that is as low as possible.
Known catalysts for steam reforming of methanol are for example the metals of subgroups 8 through 10 (Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt). They are distinguished however by poor selectivity for the desired primary reaction and lead predominantly to a breakdown of methanol to carbon monoxide and hydrogen.
Investigations with copper catalysts exhibited high catalytic activity and selectivity in steam reforming of methanol but the known copper catalysts suffer from some serious disadvantages including their pyrophoric properties upon being exposed to oxidizing conditions and the tendency of copper to sinter at temperatures above 300° C. and in that case to lose activity.
Iwasa et al., Catalysis Letters 54 (1998), pages 119 through 123, describe further catalysts for steam reforming of methanol, in which palladium or platinum is applied as a catalytically active metal to a metal oxide carrier such as ZnO, In2O3, Ga2O3, SiO2, MgO or CeO2. Isawa et al. found that the catalytic activity and selectivity of palladium and platinum were markedly improved if they are applied to the carriers ZnO, In2O3 or Ga2O3. The carriers alone exhibited no reaction.
U.S. Pat. No. 6,413,449 describes a catalyst for steam reforming of methanol, which has a palladium-zinc alloy and zinc oxide as catalytically active components on a carrier material of aluminum oxide, aluminum silicate, titanium oxide, zirconium oxide, a zeolite and mixtures or mixed oxides thereof.